Mercuric reductase is a member of the family of thiol oxidoreductases which, among others, includes glutathione reductase and lipoamide dehydrogenase. In our investigation of the catalytic mechanism of Mercuric Reductase, a homodimer that undergoes an alternate-site mechanism, one aspect involves understanding the structural changes that occur during the catalytic cycle. Recently, a crystal structure of the Bacillus sp. RC607 mercuric reductase was solved in at least two forms (with and without NADP). However, most of the biochemical studies that have been conducted focus on P. aeruginosa Tn501 mercuric reductase. The Tn501 enzyme has yet to be crystallized. RC607 and Tn501 mercuric reductases are 40% identical in their sequences. It is expected that their structures be highly homologous. The program LOOK, available through the Computer Graphics Laboratory, uses the alignment of the two sequences to ge nerate a homology model of the dimer. With the aid of CGL's workstations and programs, we were able to visualize possible interactions that may play a role in the asymmetry that develops through the catalytic cycle. From a comparison of the generated models of Tn501 enzyme against the known structure of the RC607 enzyme, we will be able to visualize similarities as well as differences that may be correlated with the biochemical information that we are also obtaining from these enzymes. We want to develop a structural model of the catalytic cycle and understand the structural pathway(s) of intersite communication. Information obtained from the homology models will allow us to construct mutant enzymes whose active site communication is disrupted.